An official with the State Department's Global Engagement Center, which monitors foreign disinformation efforts, identified four publications that he said have served as fronts for Russian intelligence.

The websites played up the vaccines' risk of side effects, questioned their efficacy, and said the US had rushed the Pfizer vaccine through the approval process, among other false or misleading claims. Though the outlets' readership is small, US officials say they inject false narratives amplified by other Russian and international media.

The Global Engagement Center official said of the sites behind the disinformation campaign all had links to Russian spooks.

 "They're all foreign-owned, based outside of the United States. They vary a lot in their reach, their tone, their audience, but they're all part of the Russian propaganda and disinformation ecosystem."

Russian state media and Russian government Twitter accounts have made overt efforts to raise concerns about the Pfizer vaccine's cost and safety in what experts outside the US government say is an effort to peddle the sale of Russia's rival Sputnik V vaccine.

A forthcoming report by the Alliance for Securing Democracy said that the emphasis on denigrating Pfizer is likely due to its status as the first vaccine besides Sputnik V to see mass use, resulting in a more significant threat to Sputnik's market dominance.

The foreign efforts to sow doubts about the vaccine exploit deep-seated anxieties about vaccines' efficacy and side effects prevalent in some US and international communities.

According to US Census Bureau data made public last month, concern about side effects is a significant reason for vaccine hesitancy.